WORKING GROUP 9 
BLESGH-TA LIANG 
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degrees of accuracy depending upon the scale and quality of the photos and 
photogrammetric equipment available. Stereo viewing alone can provide 
comparative information on topography, a parallax bar measurement can 
give spot differences in elevations and form-lining on a Kelsh plotter or com 
parable equipment can provide topographic information with relatively little 
or no field controls. 
Watershed areas and length of drainage structures can be measured and 
drain cross-sections evaluated. 
Property values can be determined with varying degrees of accuracy by 
observing types of building and land use patterns. 
Soils and geologic conditions, including borrow sources, can be assessed by 
standard photo-geologic techniques. 
Vegetation conditions can be seen directly on the photos. 
Trouble areas are detected as discussed later in this paper. 
In the preliminary location stage, large scale topographic maps become 
available and the basic photo interpretation task is a more detailed study of 
drainage, property values, soils and geology and trouble areas, including the 
design of an effective subsurface exploration program. 
During the final location stage, the line is located on large scale plans and 
staked out in the field. Soils and geology studies for structures, borrow and 
aggregate sources investigation, access road location and problems related 
to the construction program comprise the photo interpretation task. 
In summary, photo interpretation plays a very important role in the recon 
naissance stage. If good topographic information is not available, barometer 
elevations at control points plus form-lining of a few critical areas to sup 
plement stereo viewing, and topographic assessment by the photo interpreter, 
can usually provide sufficient grade data for comparing alternate corridors. 
The task of photo interpretation in the preliminary and final location stages is 
more specialized; augering, resistivity and seismic investigations as well as 
core drilling are useful supplements to the photo study of soils and geologic 
conditions. 
After all the conditions are evaluated, the importance of final mapping and 
presentation to convey the information to the designers cannot be over 
emphasized. Oftentimes, the success of a project hinges on the proper choice 
of the mapping units to present the optimum amount of pertinent information 
for location decisions. It is seldom that a mapping unit system perfectly satis 
factory for one project is equally good for another; each system should be 
designed to fit the prevailing conditions of an area. 
The Quebec Cartier Mining Railway in Canada as an outstanding 
example 
The location procedures of the recently completed 200-mile Quebec Cartier 
Mining Railway in Canada will serve as an outstanding example of application 
of photo interpretation in a major engineering project throughout its various